I've been pulling my hair out with the cursed component all afternoon, so I'm really hoping somebody can help with this.

I'm trying to setup a 301 redirect from "rentree-registration-week-sept-2011" to "accueil", and every time I type in "/rentree-registration-week-sept-2011", I am redirected to the 404 page. I have setup hundreds of redirects and they all work fine, except this one!

Any thoughts/suggestions would be most appreciated.

Hi,
Check if there's a SEF URL "rentree-registration-week-sept-2011" created on Manage SEF URLs page. If it is, delete it. Otherwise JoomSEF is trying to first use the SEF URL before it searches for the redirect. Also check that the "accueil" URL is working.
Otherwise I don't know what the problem could be, if your other 301 redirects work correctly. I can offer you our paid support to check the problem for you directly: www.artio.net/e-shop/support-services/joomla-related-support-1h